What do mangoes, cashews, pistachios, and poison ivy have in common?
Answer: A loan-word from Japanese — “urushi”, and then some.
Lacquered objects from Asia are highly treasured works of art. Lacquering was a complicated and sensitive process where the sap of the urushi tree was used to repeatedly coat the object, yielding a smooth and shiny finish. The sap from the urushi tree, or the Asian lacquer tree (Toxicodendron vernicifluum) is rich in a particular mixture of organic compounds that share a common basic structure — catechol — with additional atoms in multiple combinations; collectively they are referred to as urushiol (pronounced you-rue-she-ol). The higher the concentration of urushiol in the sap, the smoother and shinier the finish of the lacquer.
The urushi tree is a member of the Anacardiaceae family, commonly known as the cashew family; other members of the family include mango, pistachio, and poison ivy. Sumac (Rhus coriaria), a spice that is common in Middle Eastern cuisine, also belongs to the Anacardiaceae family, but lacks urushiol; poison sumac (Toxicodendron vernix) on the other hand does contain urushiol. The two sumacs belong to unique genera in the same family [a botanical family is the equivalent of a town; the genus (plural=genera) is the equivalent of a street; the species is the equivalent of your apartment number].
Urushiol is present at varying concentrations in poison ivy, mango, cashew, and pistachio. It is stored in the vascular system of these plants, and is released when the plant is either touched (poison ivy); or injured (mango, cashew, pistachio, poison ivy). It is hydrophobic (not soluble in water), and can be destroyed at high temperatures. According to the American Academy of Dermatology, “Most people (85 percent) develop a rash when they get urushiol on their skin.” However, the severity of the rash is a function of how one’s immune system reacts to the urushiol, and also the concentration of urushiol. Because of the high concentration of urushiol in poison ivy, urushiol-sensitive individuals might not react as adversely to exposure to mangoes, as they might to exposure to poison ivy. A video from the National Institute of Occupational Safety and Health (NIOSH) illustrates how urushiol causes an allergic reaction.